1. Field of the Invention
This invention relates to an apparatus for soldering printed circuit boards.
2. Description of the Prior Art
To reply to recent demands for compact electrical appliances and instruments, there is a tendency of using small thin chip type electric parts for mounting on the printed circuit boards in place of the conventional discrete type electric parts. From the view point of the parts mounting operation, the chip type parts also excel the discrete type parts in that the former can be fixed on a board by face bonding and can be mounted in positions automatically in a facilitated manner and at a lower cost, whereas the latter parts are difficult to insert into the printed circuit boards by an automatic operation and involves an increased number of steps, for example, a step for cutting off pendant lead wire portions.
Although there have thus far been proposed various soldering apparatuses for printed circuit boards, they invariably give rise to problems of soldering failures when applied to printed circuit boards bearing the chip type parts due to the recesses defined between adjacent chip type parts or behind the chip type parts as seen in the direction of travel of the boards. Such recesses or space can block the molten solder from flowing thereinto and can trap gases therein, causing incomplete deposition of the solder. The air bubbles which are once trapped in the recesses are difficult to remove even if the molten solder is poured over the printed circuit board over a long time period.
In addition, the conventional soldering technique involves a problem that the chip type parts unavoidably undergo thermal shocks when subjected to the soldering temperature. More specifically, with the conventional soldering apparatus, even if the printed circuit boards are preheated to a level proximate to the soldering temperature in a preheating zone, there would occur a material temperature drop during the transfer thereof from the preheating zone to the soldering zone. As a matter of fact, since there is a limit in the preheating temperature, it is difficult to control the temperature of the printed circuit boards as introduced into the soldering zone to a level close to the soldering temperature.